PSI - Issue 38

Yevgen Gorash et al. / Procedia Structural Integrity 38 (2022) 490–496 Y. Gorash et al. / Structural Integrity Procedia 00 (2021) 000–000

495

6

400

20kHz stopped 15Hz stopped run out in progress Δσ( U-L) FKM data GRANTA data 20kHz (s/s)

y = 651.05x -0.032 R² = 0.9783

y = 587.37x -0.03 R² = 0.9364

350

y = 594.28x -0.039 R² = 0.8521

300

y = 652.72x -0.052 R² = 0.2348

250

20kHz (p/c 2w) 20kHz (p/c 1m) Power (20kHz) Power (15Hz) Power (20kHz (s/s))

Stress amplitude (MPa)

y = 421.07x -0.049 R² = 0.9199

200

150

Power (20kHz (p/c 2w)) Power (20kHz (p/c 1m))

1.E+07 Number of cycles

1.E+04

1.E+05

1.E+06

1.E+08

1.E+09

1.E+10

Fig. 4. Summary of fatigue testing for S275JR + AS including base dry and pre-corroded samples.

100 120 140 160 180 200

30

15 18 21 24 27 30

25

20

15

4020 4040 4060

0 20 40 60 80

0 1000 2000 3000 4000 5000 6000 7000 8000

Temperature, °C

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000

Seconds

Fig. 5. Temperature history of the ultrasonic fatigue test lasted 4m cycles at 400 MPa stress amplitude.

The ultrasonic fatigue limit for S275JR + AS grade with mirror surface finish is found to be 350 MPa as confirmed by four samples in the range of 300-350 MPa that run out after 1.2 billion cycles. Three samples have demonstrated an order of magnitude longer fatigue life (see Fig. 4), and, for this reason, have been grouped and used to generate a sub-surface SN curve. The assumption of sub-surface crack origin has been investigated using optical and SEM microscopy, as shown in Fig. 6. The pre-corroded samples demonstrated significantly lower fatigue resistance with finite fatigue life of about 1 million cycles at the stress level corresponding to the fatigue limit for the ideal surface. As seen in Fig. 4, the drop of fatigue performance by 1.5 orders of magnitude in terms of cycles to failure and by over 50 MPa in terms of stress amplitude. This can be explained by the absence of the crack initiation phase in pre-corroded samples, as the crack grows directly from pits, as stress concentrators. The di ff erence between 15 Hz and 20 kHz SN curves was measured in terms of stress amplitude as 170 MPa on average and used as a basic tool for frequency correction. Scaling down the fatigue limit from 350 MPa at 20kHz using 170 MPa correction gives 180 MPa as a conservative prediction of the conventional fatigue limit at 15Hz for S275JR + AS, which is close to values from the literature (Haibach, 2003; ANSYS Inc., 2020).